Typical modern vehicle fuel systems include a fuel vapor recovery system in which fuel vapors are collected from the fuel tank to be stored in a vapor storage canister, from which they are later purged and burned in the engine. The fuel tank has a fuel vapor colelction dome at the top, through which is connected a fuel vapor vent line that runs to the vapor storage canister. Should the tank be overfilled, there is the possibility that liquid fuel could contaminate the vapor storage canister through the vent line. This could also occur in the event of vehicle rollover, or even excessive tilting of the vehicle from the horizontal.
The prior art shows several examples of a basic protection valve design to protect against each possibility. The basic design consists of a fuel receiving chamber inside the tank and located beneath the vent line opening. A ball shaped or conical weight sits within the chamber, resting on a conically shaped lower wall of the chamber. A float sits between the weight and the vent line opening, resting on the top of the weight spaced from the vent line opening. When the vehicle tank has a normal fuel level and is not tilted excessively from the horizontal, the vent line opening is unblocked, and fuel vapors that form in the tank are collected in the dome, exiting through the vent line to the canister. Should the fuel tank be overfilled, the float moves up off of the weight with the rising fuel, and is guided by the interior wall of the chamber into sealing engagement with the vent line opening, to prevent the escape of liquid fuel. Should the vehicle tilt sufficiently that the conical lower wall of the chamber moves beyond the horizontal, then the weight can roll or slide along the conical wall to push the float into the same sealing engagement.
While the basic design described above works, it does have some design limitations. It is desirable that the valve not close off unless the vehicle has tilted to excess, so that the tank may vent freely when the vehicle is in a normal attitude. The angle of tilt beyond which it is desired to shut off the vent line, which may be referred to as the closing angle, in turn determines the angle of the conical lower chamber wall. The basic valve described will not respond at all until the conical wall has moved beyond the horizontal, at which point the valve closes as quickly as the weight can roll or slide along the chamber conical wall. It is desirable also, for good venting, that the float sit sufficiently below the vent line opening that the fuel vapors have good access to the opening, and so that the float will not bounce up intermittently and close the opening off. This may be referred to as the float's normal distance. Consequently, the weight must move toward the vent line opening at least that normal distance in order to in turn push the float against the opening. Since the conical wall has a predetermined angle, it must be also sufficiently wide to allow the mass to move that far, so there is an inherent limit on how compact it may be. Providing a different closing angle would also require a differently shaped and potentially wider conical wall.